High-powered optical sources have many applications in which an intense beam of light is focused onto a substrate or other target. Many high power optical systems make use of wavelength conversion to produce light having a desired wavelength or range of wavelengths. Often the process of conversion involves performing some non-linear optical wavelength conversion on input light from a source, such as a laser. The wavelengths that can be produced by nonlinear optical wavelength conversion are limited however by the wavelengths that can be produced with available lasers and the nonlinear optical wavelength conversion processes. For example, many wavelength-converted laser systems are based on a seed laser that produces light at a fundamental vacuum wavelength of 1064 nanometers. The infrared 1064 nm light can be converted to 532 nm visible light by nonlinear frequency doubling. The 523 nm visible light can be converted to 266 nm ultraviolet light in a subsequent nonlinear frequency doubling process.
Optical frequency conversion often requires high peak input intensity (power per unit area) in order to obtain efficient optical frequency conversion. As a result, the output beam is often a narrow, high power beam. It is often desired to expand or otherwise shape the narrow output beam. However, high intensity, high frequency output beams present challenges for beam shaping. Beam shaping is typically done with mirrors, lenses, or other optical components that expand the beam diameter and/or convert an elliptical beam to a round beam. One of the challenges is to reduce reflections of the output beam from the surfaces of the optical elements used for beam shaping. One conventional approach to reducing reflections is to coat the surfaces of the optical elements with an anti-reflection (AR) coating. Unfortunately, such coatings are susceptible to damage from high intensity, high frequency light.
It is within this context that embodiments of the present invention arise.